To have detailed knowledge of all the complexities, intricacies, and nuances of a particular subject or situation.
This phrase dates to the 1600s and originally referred to knowing all the turns, corners, and passages of a place - literally the ways in and the ways out. It evolved from physical navigation to metaphorical expertise about any complex system or topic.
The beauty of this phrase lies in its architectural metaphor - just as knowing a building's ins and outs means understanding both obvious entrances and hidden passages, true expertise involves grasping both the apparent and obscure aspects of any field.
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